I've skipped using Irish moss once and ended up with a cloudy bitter. That was also my first, and only, use of gelatin to try to drop some haze. But I do often cut back on the amount of IM and it still seems to work.

I like using Whirlfloc tablets which are a refined type of Irish Moss and can be tossed in 15 minutes before the end of the boil.

As Martin states they are not a panacea but are quite effective and very easy to use.

Manufacturer recommends 5 m in. before the end of boil. One tab is good for 12 gal., so 1/2 tab is fine for a 5 gal. batch. There's a lot of confusion about this because so many retailers repackage Whirlfloc with inaccurate instructions.